Efficiency of two-phase designs for prevalence estimation

R. McNamee

    Research output: Contribution to journalArticlepeer-review


    Background. Unbiased estimation of the prevalence of diseases and other conditions is important but can be expensive, especially for conditions which do not necessarily lead to contact with health services. A two-phase population survey may seem an attractive option when there is a relatively cheap, although fallible, test for disease status available: the test is used in the first phase of the survey but in the second, only a subsample are classified by the relatively expensive, gold standard. Previously the cost efficiency of such studies compared with simple, one-phase random sample designs was investigated empirically and some questions remain unclear. Methods. A simple formula for the maximum reduction in cost or standard error that can be achieved by two-phase sampling compared with simple random sampling is derived mathematically. A formula for the minimum reduction is also given and the influence of prevalence on efficiency explained. Results. The main result shows that the sensitivity and specificity of the first stage test set an absolute limit on the efficiency of two-phase designs; in particular, two-phase sampling can never be justified on efficiency grounds alone if the test is not accurate enough.
    Original languageEnglish
    Pages (from-to)1072-1078
    Number of pages6
    JournalInternational Journal of Epidemiology
    Issue number6
    Publication statusPublished - Dec 2003


    • Cost efficiency
    • Double sampling
    • Epidemiology
    • Prevalence estimation
    • Statistical efficiency
    • Survey design
    • Two-phase designs
    • Two-stage designs


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